You probably didn't whip the meringue part stiff enough. Try adding a couple of Tablespoons of cornstarch first, before adding any more powdered sugar. The powdered sugar will start to make it crusting after a while.

Hi - I have made this frosting a couple of times with success, but recently it just doesn't seem to fluff up like it used to, its kind of a thick runny frosting if that makes sense - kind of cake batter texture, and i am not changing anything. I whipped it to very stiff peaks which is why i'm confused. Any idea what could be the problem? i have added a little less powdered sugar this time as I am about 80g short and its too late to go out to the shops - could that be it?!